The present invention relates to an inkjet recording head, and particularly to an inkjet recording head capable of high speed and stable drive by compensating variation of ink ejection speed from each channel due to crosstalk generated at the time of drive.
Conventional Technology
Various methods have been proposed for an inkjet recording head, and one of these is an inkjet recording head of a shear mode (Patent Literature 1).
FIGS. 1 and 2(a), (b) are drawings to show an example of this inkjet recording head. FIG. 2(a), (b) are partial cross-sectional views taken on line Z—Z in FIG. 1. Number 1 is an ink tube, 2 is a nozzle forming member, 3 is a nozzle, 4 is an ink channel, 5 is a sidewall, 6 is a cover plate, 7 is an ink supply opening, 8 is an electrode and 9 is a substrate. As can be seen from FIG. 1 and FIG. 2(a), ink channels 4 are constituted by sidewalls 5, cover plate 6 and substrate 9, and the ink channels 4 have a flat bottom portion and a curved bottom portion. The shape of this inkjet recording head is an example of a preferred embodiment, and is not restricted to the shape shown in FIG. 1.
Many ink channels 4 which are separated by a plurality of sidewalls 5 are constituted between cover plate 6 and substrate 9, as shown in a cross-sectional view of FIG. 2. In FIG. 2, only three of a plurality of ink channels 4 are shown. One end of ink channel 4 is connected to nozzle 3 which is formed in nozzle forming member 2, and ink channel 4 is connected to an ink tank, which is not shown in the drawing, by ink tube 1 via ink supply opening 7. Further, electrodes 8a, 8b and 8c, which extend from the upper portion of both sidewalls 5 to the bottom face of substrate 9, are adhered on sidewall 5 in each ink channel 4. Each of the electrodes 8a, 8b and 8c connects the respective electrodes, opposing each other and facing the inside of ink channels 4, in common as shown in the drawing, and an ink drop is ejected according to the following movement when a printing pulse is applied on said electrodes opposing each other.
Sidewall 5 is constituted of sidewalls 5A and 5B comprising two piezoelectric substances having different polarization directions, sandwiching an adhesive portion, as shown by arrows in FIG. 2(a). Sidewalls 5A and 5B do not deform when a printing pulse is not applied on any of electrodes 8a, 8b and 8c, while generated is an electric field in the perpendicular direction to the polarization direction of a piezoelectric substance, resulting in causing shear deformation at an adhesive face between sidewalls 5A and 5B, when a printing pulse is applied on electrode 8a as shown in FIG. 2(b) and electrodes 8b and 8c are simultaneously grounded, thereby pressure of ink is changed to eject a part of ink filling ink channel 4 from nozzle 3. Herein, the direction of deformation of a sidewall can be changed by changing the polarity of a printing pulse and the direction of electric field thereby. Hereinafter, the movement of applying a pulse to electrodes opposing each other, which are connected together to face the inside of ink channel 4, is expressed as “to apply a pulse to a channel”. In FIG. 2(a), (b), a nozzle is not shown.
Driving this inkjet recording head of a multi-channeled shear mode is generally performed by dividing ink channels 4 into 3 groups to be driven in turn in a time-sharing mode. Hereinafter, in this description, this time-sharing may be referred to as “period” and the time-sharing of an ink channel divided into n parts as “n-period”. In the embodiment shown in FIG. 3, an inkjet head will be explained as the ink channels are constituted of 9 channels of A1, B1, C1, A2, B2, C2, A3, B3 and C3. Further, the time chart of printing pulses is shown in FIG. 4. In FIG. 4, a pulse wave shape applied to each ink channel is expressed vertically and each period (time) horizontally, however, scales of such as time and pulse voltage is not always expressed correctly.
As shown in FIG. 3(a), when printing pulse Pa (shown in FIG. 4) is applied to drive A group, three channels A1, A2 and A3 simultaneously, at the first period T1a, sidewalls of these three channels A1, A2 and A3 are deformed simultaneously resulting in ejection of ink drops from each nozzle. In a similar manner, as shown in FIGS. 3(b) and 3(c), when printing pulse Pb (shown in FIG. 4) is applied to drive B group, three channels B1, B2 and B3 simultaneously, at the second period T1b, and printing pulse Pc (shown in FIG. 4) is applied to drive C group, three channels C1, C2 and C3 simultaneously, at the third period T1c, each sidewall is deformed successively to drive all of 9 channels by circulating a sequence of three periods, T1a, T1b and T1c, ejection of ink drops from each nozzle results.
It is clear from FIGS. 3 and 4 that 9 ink channels are divided according to the arrangement order into units U1, U2 and U3, each of which contains three ink channels comprising each one ink channel belonging to A group, B group and C group, and are driven at a drive cycle comprised of periods T1a, T1b and T1c. Images are formed by repeating this drive cycle. In the embodiment of FIGS. 3 and 4, three ink channels constitute one unit, however, n (n≧2) ink channels generally constitute one unit and applied is a driving method in which n periods constitute one drive cycle.
Naturally, in the aforementioned driving method, a printing pulse is not necessarily applied to all ink channels as described above and some ink channels are not driven depending on image signals when images are practically formed.
<Patent Literature 1>
Japanese Patent Publication Open to Public Inspection No. 2-150355
Problems to be Solved
As explained above, it has been proved that when driven at 3 periods is a shear mode inkjet recording head, in which many sets of a plurality of ink channels are arranged, sidewall 5 is deformed to transmit a part of the pressure and to affect other ink channels resulting in crosstalk between a driven ink channel and other ink channels, which in turn results in varying ejection speed of ink drops to cause undesirable effects on image quality.
As described above, three channels of A1, A2 and A3 belonging to A group are driven simultaneously at first period T1a. In this case, due to symmetrical effect, the pressure variation in ink channels B1, C1, B2, C2, . . . is half value with opposite sign (positive or negative) to the pressure variation in ink channels A1, A2, . . . . On the other hand, in the case where ink channel A2 is singularly driven, the pressure variation extends farther to C1, B1, A1, B1, C2, A2, . . . . As the result, the pressure generated in A2 is greater in the case where A1, A2, and A3 are simultaneously driven than in the case where A2 is singularly driven. Thereby ink channel A2, when simultaneously driven, ejects ink drops at a higher speed resulting in variation of size and shape of ink drops.
This phenomenon is also observed with ink channels A1 and A3, by getting effects mutually from ink channel A0 which is located at the left side of ink channel A1, and ink channel A4 which is located at the right side of ink channel A3, although they are abbreviated in the drawing, resulting in so-called crosstalk, and ink drops are ejected at a high speed from all the ink channels belonging to A group except ink channels at the both end when all the ink channels in A group are driven in this way. However, as shown in FIG. 5, when only ink channel A2 is driven, ink ejected from ink channel A2 shows slower speed than that when ink channels A2 is driven simultaneously with A1, A3, . . . , which may cause the volume change of ink drops resulting in undesirable problems in image formation. In practice, the effects of crosstalk, which individual ink channels receive, differs depending on image signal patterns, and speed and volume of ink drops ejected from nozzles differ depending on individual states.
Further, the range of ink channels in which this crosstalk is caused depends on rigidity of a material comprising ink channels, however, generally crosstalk transmits as far as the range of several channels. Therefore, the spacing between ink channels which drive simultaneously may be extended and a number of driving period is increased, for example, to drive at 6 periods may be preferred, however, there causes problems of such as prolonged total image forming time.
This invention is presented to solve the problem of the effects on other channels by crosstalk caused at the time of driving, and the objective is to provide an inkjet recording head in which variation of the ejection speed from each ink channel due to crosstalk is compensated, and capable of high speed and stable driving as well as highly visible image formation.